1. Field of the Invention
The present invention relates generally to a wavelength division multiplexing optical communication system, and more particularly to a multi-wavelength light source unit capable of providing a plurality of light signals having different wavelengths.
2. Description of the Related Art
Conventional wavelength division multiplexing (WDM) communication systems generate multiplexed light signals from a plurality of light channels. The multiplexed light signal has extremely narrow wavelength spacing between the light channels. This allows for the transfer of more light signals on a predetermined optical communication wavelength band. Generally, it is known in the art that the WDM optical communication system serves to concurrently transfer a plurality o f multiplexed light beam signals with different wavelengths on a single optical fiber. One or more semiconductor laser devices, e.g. laser diodes, have been used as a light source for emitting light beams.
However, such semiconductor laser devices usually have very high sensitivity to changes in temperature. This has a critical disadvantage in that the oscillation frequency slightly fluctuates to changes in the ambient temperature. Since the wavelengths of demodulated light beams are positioned very close to each other, even a minor error in wavelength can cause interference with each other. For instance, when the semiconductor laser device fails to radiate light beams in its intended precise wavelength band owing to a change in its ambient temperature (or for any other reason), an optical receiver may be unable to identify those light beam signals in full due to light diffusion, loss of light or the like, which often inherently occurs in the course of transmission of light beams carried on the optical fiber. Further, while the light source may be able to precisely control the wavelength initially, it is usually very difficult to ensure long-term stability in the wavelength throughout its lifetime use.
Another example of a conventional light source device used in a WDM optical communication system is a multi-wavelength light source. Such light source devices include a plurality of light source units, each unit providing a different wavelength of light beam, and a set of multiplexer/demultiplexer for multiplexing or demultiplexing the light beam signals out of the light source units. A semiconductor laser device may be used as a light source for use in the multi-wavelength light source.
U.S. Pat. No. 6,545,784 (Optical cross connect unit, optical add-drop multiplexer, light source unit, and adding unit) to Okazaki, et al. discloses an apparatus for modulating a given wavelength of lights from a number of light sources into multiple lights so as to generate a plurality of lights having different wavelengths as desired. The above apparatus includes a plurality of light sources, an optical add/drop section for demultiplexing a plurality of different wavelengths of lights outputted from the light sources, and a plurality of filters for splitting the plurality of lights into respective different wavelengths of lights. The optical cross connect unit and multi-wavelength light source according to the apparatus of Okazaki, et al. further includes a plurality of modulators for modulating a plurality of light signals output from the multi-wavelength light source into electrical signals and then modulating the electrical signals into a predetermined wavelength of lights, thereby generating a desired wavelength of light signal. It renders a plurality of lights having different wavelengths out of a small number of light sources.
However, it is noted that the above-mentioned multi-wavelength light source unit requires an external modulating means for carrying out modulation of the optical signals into electrical signals to generate a plurality of lights having predetermined different wavelengths. As a result, such an additional modulator has disadvantageous effects, e.g., an increase in the overall volume of optical apparatus and increase in production cost.
Accordingly, there is a need in the art for an improved multi-wavelength light source.